1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount which is capable of damping input vibrations based on flows of the fluid enclosed therein, and in particular to such a fluid-filled elastic mount which exhibits improved vibration-damping characteristics for vibrations in a comparatively high frequency range.
2. Discussion of the Prior Art
A mounting member such as an automotive engine mount is generally required to exhibit an excellent vibration-damping effect against input vibrations in a wide frequency range, particularly excellent vibration-damping characteristic against low-frequency large-amplitude vibrations and a sufficiently low dynamic spring constant or rate with respect to high-frequency small-amplitude vibrations.
In the light of the above requirements, there has been recently proposed a fluid-filled elastic mount of a type as disclosed in German Laid-Open Patent Application DE 30 19 337 Al. This fluid-filled elastic mount includes: (a) a first and a second support member which are spaced apart from each other in a load-receiving direction in which a vibrational load is applied to the elastic mount, (b) an elastic body for elastically connecting the first and second support members, (c) means for defining a pressure-receiving chamber between the first and second support members, the pressure-receiving chamber being filled with a non-compressible fluid and receiving the vibrational load applied to the elastic mount, (d) means for defining an equilibrium chamber between the first and second support members, the equilibrium chamber being filled with the non-compressible fluid, at least partially defined by a flexible diaphragm and having a variable volume, (e) means for defining a restricted passage for fluid communication between the pressure-receiving and equilibrium chambers, and (f) a movable member disposed between the pressure-receiving and equilibrium chambers such that the movable member is movable by a predetermined distance in the load-receiving direction so as to absorb a fluid-pressure difference between the pressure-receiving and equilibrium chambers.
The fluid-filled elastic mount of the type described above had been developed based on the knowledge that low-frequency vibrations have large amplitudes, for which an elastic mount is required to exhibit excellent vibration-damping characteristic, while high-frequency vibrations have small amplitudes, for which an elastic mount is required to exhibit a sufficiently low dynamic spring constant. Upon application of low-frequency large-amplitude vibrations to the known elastic mount, the movements of the movable member in the load-receiving direction cannot accommodate or absorb a change in the fluid pressure of the pressure-receiving chamber, on the other hand the restricted passage (i.e., orifice) properly serves or operates to permit the fluid to flow therethrough so as to absorb the fluid-pressure change and thereby damp the input vibrations. This vibration-damping characteristic is proper to the orifice. Meanwhile, upon application of high-frequency small-amplitude vibrations, the movements of the movable member result in substantial fluid flows between the pressure-receiving and equilibrium chambers, thereby preventing an increase in the fluid pressure of the pressure-receiving chamber, namely avoiding an increase in the dynamic spring constant of the elastic mount due to a substantially closed condition of the orifice. Consequently the elastic mount exhibits a lowered dynamic spring constant for the input vibrations.
In the above fluid-filled elastic mount, it is preferred that the orifice have a sufficiently large cross-sectional area for the fluid flows therethrough and a sufficient length for the fluid flows therealong, to provide excellent damping characteristic for vibrations in a comparatively low frequency range. The above-indicated German Patent Application teaches forming the orifice in an outer peripheral portion of a partition member which separates the pressure-receiving and equilibrium chambers from each other, such that the orifice extends in the circumferential direction of the partition member. Meanwhile, the above-indicated movable member is movably supported by an inner or central portion of the partition member which is surrounded by the orifice.
In the above fluid-filled elastic mount, however, the effective area of the movable member, that is, cross-sectional area surrounded by the central portion of the partition member which area is effective to displace the fluids in the pressure-receiving and equilibrium chambers based on the movements of the movable member, is limited to a considerably small region because of the provision of the orifice. Consequently, the inner or central portion of the partition member defining the above-indicated cross-sectional area, is placed in an unoperable condition as if it is closed, upon application of vibrations in a relatively low frequency range, whereby the movable member is not capable of providing a satisfactory, fluid pressure change-absorbing effect for vibrations in a relatively high frequency range.